The present invention relates to integrated optical devices, and also to methods of making such devices.
Integrated optical devices generally include a substrate formed with waveguide pathways each having a higher refractive index than the substrate for guiding the transmission of light therethrough, and a plurality of electrodes to receive electrical signals for controlling the light transmission through the pathways. The waveguide pathways in an interaction zone are of an electro-optically active waveguide material whose refractive index changes in response to electrical signals applied to the electrodes. Waveguide pathways in an access zone provide optical access to the interaction zone.
The invention is particularly useful in cavity-assisted directional-coupler devices in which the interaction zone includes an optical cavity having front and back ends defined by reflector facets perpendicular to the longitudinal axis of the optical cavity. The invention is therefore described below particularly with respect to this type of device, but it will be appreciated that the invention, or various aspects thereof, may also be used in other types of integrated optical devices.
Integrated optical devices are characterized by extremely short response times, in the sub-nano-second order, which makes them ideally suited in optical communications systems. Such devices generally, and cavity assisted directional-coupler devices in particular, are described in a large number of publications, including the Ph.D. thesis by the inventor in the present application: D. Nir, xe2x80x9cNovel Integrated Optic devices Based On Irregular Waveguide Featuresxe2x80x9d, Ph.D. thesis, Tel Aviv University, 1996.
The extension of such devices to ever-increasing applications depends to a large degree on the operational efficiency attainable by such devices, and also on the complexity in fabricating such devices. Efforts are continuously being made to increase the operational efficiency of such devices, and to simplify their fabrication, in order to extend their use to many additional applications.
For example, a fundamental feature of cavity-assisted directional-couplers is a very short optical cavity, typically 25-250 xcexcm in length. The cavity is created when two reflectors confine a waveguide section.
The reflector structures, in particular at the input side, are generally trench structures created by etching out material, as by reactive-ion-beam etching (RIBE). The reflector facets must be perfectly flat, smooth and perpendicular to the optical cavity in order to minimize cavity losses because of scattering by imperfections. The back facet of the front trench is coated with a semi-reflecting film to input the light, whereas the front facet in the back trench is coated with a fully reflecting film to produce total reflection through the optical cavity between the latter two films.
Because of the trench structure produced by etching, the front facet of the front trench (facing the input waveguide) is coated with an anti-reflecting film to improve the light transmission. However, providing such a film adds to the complexity of fabrication; it also contributes to the optical losses in such devices.
An object of the present invention is to provide integrated optical device of the foregoing type, and methods for making them, to improve the operational efficiency of the devices, and/or to reduce the complexity in their fabrication.
According to one aspect of the present invention, there is provided an integrated optical device, comprising: a substrate including waveguide pathways each having a higher refractive index than the substrate for guiding the transmission of light therethrough, and a plurality of electrodes to receive electrical signals for controlling the light transmission through said pathways;
the waveguide pathways being included in an interaction zone and being of an electro-optically active waveguide material whose refractive index changes in response to electrical signals applied to the electrodes; the waveguide pathways also being included in an access zone providing optical access to the interaction zone; characterized in that the active waveguide material in the interaction zone is a different material from the waveguide material in the access zone.
As will be described more particularly below, this broad aspect of the invention enables a number of techniques to be used for improving the operating efficiency of such devices, as well as for reducing the complexity in their fabrication.
According to another aspect of the present invention attainable by the above feature, there is provided an integrated optical device of the optical cavity type characterized in that the reflector facets for the optical cavity (or cavities) are defined by trenchless formations in the substrate and consist only of a semi-reflecting facet at the front end of the optical cavity and a fully-reflecting facet at the back end of the optical cavity. Such a construction, obviating the need for trenches and an anti-reflecting facet at the inlet end of the optical cavity, not only enables the operation efficiency of the device to be improved by eliminating optical losses in the anti-reflecting coating on the front facet of the front trench, but also enables the fabrication of such devices to be simplified.
According to another aspect of the present invention also attainable by the foregoing feature the invention provides cavity-assisted directional couplers including a single optical cavity on the interaction zone, characterized in that both the input waveguide pathway and the output waveguide pathway are coupled to the optical cavity on the same side of the substrate, as distinguished from the prior art constructions, as described below (and illustrated in FIG. 1a) wherein they are on opposite sides of the substrate. Such a feature may be highly desirable in many designs to increase the flexibility and/or compactness of the design.
According to another aspect of the present invention, there is provided a method of making a cavity-assisted directional-coupler in which the interaction zone includes an optical cavity having front and back ends defined by reflector facets perpendicular to the longitudinal axis of the optical cavity produced by dicing and polishing, rather than by precise etching. As will be described more particularly below, such a method enables attaining both an increase in the operating efficiency of the device, as well as a reduction in the complexity of its fabrication.
According to another aspect of the present invention, there is provided an integrated optical device characterized in that a second substrate is bonded to the substrate formed with the interaction zone waveguide pathways and is of a material having a higher heat capacity than the material of the latter substrate so as to serve as a heat sink for that substrate. Such a construction permits the substrate including the waveguide pathways to be made of a first material, such as LiNbO3, having a relatively low heat capacity and a relatively high thermal sensitivity, and the second substrate to be made of a material, such as silicon, having a high heat capacity so as to serve as a heat sink for the first substrate and thereby to minimize its temperature change during the operation of the device.
According to a still further aspect of the present invention, there is provided a method of producing an integrated optical device including waveguide pathways defining an optical cavity of an interaction zone, and waveguide pathways in an access zone; the method comprising: forming the waveguide pathways of one zone in a first substrate; bonding the first substrate to a second substrate to embed the waveguide pathways; etching one of the substrates to produce perpendicular facets at the front and back ends of the optical cavity of the interaction zone; and applying reflector coatings to the perpendicular facets.
In the preferred embodiment of the invention described below, the second substrate is silicon and is etched to form a mask for etching the first substrate to produce the waveguide pathways of the interaction zone, and particularly the perpendicular facets at the opposite ends of the optical cavity. Since silicon is easily etchible by conventional wet etching techniques, as distinguished from LiNbO3 which generally requires reactive ion beam etching (RIBE), this aspect of the invention enables relatively perfect reflector facets to be produced at the opposite ends of the optical cavities by wet etching rather than by RIBE.
Further features and advantages of the invention will be apparent from the description below.